1. Field of the invention
The present invention relates to an inclusion decanting process for metallic materials, particularly nickel-based superalloys.
The techniques normally used nowadays for the processing of metallic materials such as nickel-based superalloys involve melting operations in crucibles made of a ceramic type refractory material and carried out under vacuum in a furnace. During such operations a metal/ceramic reaction occurs, which inevitably results in the presence of ceramic inclusions in the material obtained. Refining the metal thus becomes necessary each time the conditions of use demand that a so-called superclean alloy should be obtained, and this is particularly the case with nickel-based superalloys intended for aeronautical applications, such as in the manufacture of parts for aeronautical turbine engines or other propulsion units. For example, in some cases, it may be desired to obtain ingots intended for the production of nickel-based powders, with a view to manufacturing parts by known powder metallurgical techniques. It is reconized that the presence of inclusions in such parts is a factor detrimental to their operating performance, especially when the parts are subject to oligocyclic fatigue stresses.
2. Summary of the prior art
To achieve the necessary refining of the superalloy, various methods have been proposed involving remelting the material in clean conditions and in a manner such as to ensure inclusion separation.
It is thus known to use a cooled crucible in which the superalloy acts as a decanting crucible for the liquid metal, melting being effected by means of an electron beam or plasma beam.
However, these methods involve a delicate procedure in plant which is often very complex and costly. In addition, depending on the intended use of the product, the effectiveness of the inclusion separation achieved is sometimes inadequate.
To solve these problems without suffering the drawbacks of the previously known solutions, the invention proposes an application of the principles of magnetohydrodynamics to liquid metals.
Examples of the application of these principles, particularly in applying an electromagnetic field to a flow of liquid metal, are disclosed in FR-A-2 316 026, FR-A-2 396 612, FR-A-2 397 251, FR-A-2 457 730 and EP-A-0 083 898.
Also, FR-A-2 452 958 describes an electromagnetic device for the separation of inclusions contained in an electrically conducting fluid in which the alternating magnetic field reacts with external currents, induced in a ring of liquid metal. However, solutions of this type cannot be exploited industrially in the case of the remelting of nickel-based superalloys for which the melting temperatures are in excess of 1300.degree. C., and superclean conditions are demanded.
Devices or processes envizaged, for example, in FR-A-2 561 761 or EP-A-0 234 536, provide for evacuation of inclusions situated at the top of a cold crucible. However, these arrangements are found to be incompatible with industrial applications in which the continuous feeding of the metal to be remelted is effected from the top of the crucible.